1. Field of the Invention
The present invention relates to a color cathode ray tube (CRT), and more particularly, to a shadow mask assembly of a color CRT through which electron beams emitted from an electron gun are passed.
2. Description of Related Art
In a color CRT, a shadow mask is separated at a predetermined distance from and opposite a fluorescent film, and an electron beam emitted from an electron gun passes passing an electron beam passing hole formed in the shadow mask and then collides with the fluorescent film. Accordingly, the shadow mask passes electron beams according to R, G and B, thus performing a color selection function.
As shown in FIG. 1, a shadow mask assembly is comprised of a shadow mask 21 and a frame 22 for supporting the shadow mask 21. The shadow mask 21 includes a mask plate 21a having a plurality of electron beam passing holes 10 formed therein, a rim portion 21b which surrounds the mask plate 21a, and a skirt portion 21c which extends perpendicularly downward from the rim portion 21b. Also, the frame 22 includes a support wall 22a welded with the skirt portion 21c of the shadow mask 21, and a flange portion 22b extending from the support wall 22a inward.
During operation of the CRT, only about 15 to 30% of the electrons, i.e., electron beams emitted from an electron gun (not shown), passes passing the electron beam through holes 10 of the shadow mask 21, while most collide with the mask plate 21a. Accordingly, the shadow mask 21 and the frame 22 are heated by collisions with the electrons, and thus expand thermally. Here, the radius of curvature of the mask plate 21a varies due to a difference in thermal expansion between the shadow mask 21 and the frame 22 according to time. That is, during the initial stages of operation of the CRT, the shadow mask 21 is heated prior to the frame 22 and then expanded, which generates a doming effect in which the radius of curvature of the mask plate 21a decreases. Thereafter, the frame 22 is also heated and then expanded with time so that the radius of curvature of the mask plate 21a increases again.
The variation in the radius of curvature of the mask plate 21a causes a change in position of the electron beam passing holes 10 formed in the mask plate 21a, which results in an inaccurate landing of electron beams emitted from the electron gun onto a fluorescent body.
In the prior art, a scheme for compensating for the thermal expansion of the shadow mask assembly by varying the interval between the shadow mask assembly and the fluorescent film according to time is used, but does not provide a sufficiently satisfactory result.